1. Technical Field
This invention relates to capacitance probes, and more particularly, to a capacitance probe and spacer for spacing conductors of the probe.
2. Background Information
Capacitance probes are often used to measure the level of a material in a tank or other compartment. As the material rises in the compartment, it replaces the air between two electrodes or conductors. If the material has a higher dielectric constant than air, the total capacitance of the system is increased as the compartment is filled. This increase in capacitance provides an indication of the amount of material in the compartment.
In order for capacitance probes to operate in this setting, a pair of conductors must be spaced such that the material to be measured may fill the space therebetween. If the probe is to be inserted into conductive materials, it must also incorporate some method of electrically insulating the conductors from each other.
One capacitive apparatus, shown in U.S. Pat. No. 3,774,238 to Hardway, uses two long tubes or rods 26, 27 insulated from each other in a spaced apart relationship by plastic insulators 28. Unfortunately, this type capacitance probe suffers from the problem of being too large and/or physically unsteady for some applications.
Another type of capacitance probe, shown in U.S. Pat. No. 5,397,995 to Anderson, includes an outer conductor and a spaced inner conductor. The space between conductors insulates the conductors from one another and allows the material to be measured to fill the space. A number of plastic insulating discs 65 are located at spaced intervals within the space to center and maintain the concentric relationship of the inner conductor within the outer conductor. The construction of this type probe is complicated by the need to assure the discs fit within the outer conductor and assure even spacing along the inner conductor. Accordingly, this type probe is costly to produce.
A need thus exists for an improved capacitance probe and spacer therefor that is more stable and cost-efficient to produce.
According to a first aspect of this invention, a capacitance probe is provided having an outer conductor having apertures extending through an outer surface thereof, an inner conductor located within the outer conductor; and at least one spacer having a member substantially surrounding the outer surface of the outer conductor and a spacing element extending from the member through each aperture to position the inner conductor. This aspect may also include, inter alia, a fitting including an O-ring for securing the conductors to a housing. The fitting and O-ring provide a simple low cost method to securely attach the conductors to the housing, and seal electronics within the housing from contamination. A non-conductive mount that isolates the probe from the compartment and reduces stray capacitance may also be provided.
In a second aspect of the invention is provided a spacer for spacing an outer conductor and an inner conductor of a capacitance probe, the spacer having: a member substantially surrounding an outer surface of the outer conductor; and at least two spacing elements extending inwardly from the member, whereby each spacer extends through an aperture in the outer conductor to position the inner conductor.
The invention provides, in a third aspect, a capacitance probe having a housing; an inner conductor made from stainless steel tubing; an outer conductor made from stainless steel tubing, the outer conductor having apertures extending through an outer surface thereof; a fitting for coupling the conductors to the housing, the fitting including an O-ring for sealing the inner and outer conductors to the housing; and at least one spacer having a member substantially surrounding the outer surface of the outer conductor and a spacing element extending from the member through each aperture to position the inner conductor.
In a fourth aspect of the invention is provided a capacitance probe fitting for coupling an inner conductor and an outer conductor of a capacitance probe to a housing. The fitting includes: a flared surface on an end of the outer conductor; an O-ring seat on the inner conductor; a fastener having a flared inner surface for mating with the flared surface of the outer conductor and coupling the outer conductor to the housing; and an O-ring positioned in the O-ring seat and the flared surface of the outer conductor for sealing both the inner and outer conductors to the housing.
Spacers that are installed from the outside of the outer conductor through apertures in the outer conductor simplify production of a capacitance probe. Further, the spacers allow material to fill and drain easily between the conductors, and allow more of the material being measured to make contact with the conductors improving linearity and gain of the measurement.
The above and other features and advantages of the invention will be more readily apparent from a reading of the following more detailed description of various aspects of the invention taken in conjunction with the accompanying drawings.